mysql select column default value if is null

 SELECT
  `w`.`city`            AS `city`,
  `w`.`city_en`         AS `city_en`,
  `w`.`date_y`          AS `date_y`,
  `w`.`week`            AS `week`,
  `w`.`cityid`          AS `cityid`,
  `w`.`temp1`           AS `temp1`,
  `w`.`temp2`           AS `temp2`,
  `w`.`temp3`           AS `temp3`,
  `w`.`temp4`           AS `temp4`,
  `w`.`temp5`           AS `temp5`,
  `w`.`temp6`           AS `temp6`,
  `w`.`weather1`        AS `weather1`,
  `w`.`weather2`        AS `weather2`,
  `w`.`weather3`        AS `weather3`,
  `w`.`weather4`        AS `weather4`,
  `w`.`weather5`        AS `weather5`,
  `w`.`weather6`        AS `weather6`,
  `w`.`wind1`           AS `wind1`,
  `w`.`wind2`           AS `wind2`,
  `w`.`wind3`           AS `wind3`,
  `w`.`wind4`           AS `wind4`,
  `w`.`wind5`           AS `wind5`,
  `w`.`wind6`           AS `wind6`,
  `w`.`fl1`             AS `fl1`,
  `w`.`fl2`             AS `fl2`,
  `w`.`fl3`             AS `fl3`,
  `w`.`fl4`             AS `fl4`,
  `w`.`fl5`             AS `fl5`,
  `w`.`fl6`             AS `fl6`,
  `w`.`ind`             AS `ind`,
  `w`.`index_xs`        AS `index_xs`,
  `w`.`index_uv`        AS `index_uv`,
  `w`.`index_xc`        AS `index_xc`,
  `w`.`index_tr`        AS `index_tr`,
  `w`.`index_co`        AS `index_co`,
  `w`.`index_cl`        AS `index_cl`,
  `w`.`index_ls`        AS `index_ls`,
  `w`.`index_ag`        AS `index_ag`,
  `w`.`create_time`     AS `create_time`,
  `sk`.`temp`           AS `temp`,
  `sk`.`wd`             AS `wd`,
  `sk`.`ws`             AS `ws`,
  `sk`.`sd`             AS `sd`,
  `sk`.`wse`            AS `wse`,
  `sk`.`time`           AS `time`,
  `sk`.`create_time`    AS `create_time_sk`,
  COALESCE(`air`.`aqi`,0)           AS `aqi`,
  COALESCE(`air`.`level`,'')         AS `level`,
  COALESCE(`air`.`position_name`,'') AS `position_name` 
FROM (SELECT   * FROM  `ep_weather_report_current`    WHERE `city`='汝州') w
  LEFT JOIN  `ep_weather_sk_temperature_current` `sk` ON `w`.`cityid` = `sk`.`cityid`
  LEFT JOIN   (SELECT * FROM `ep_airpollution_realtime_current` WHERE `city` = `position_name` AND  city = '汝州') `air`
  ON `w`.`cityid` = `air`.`cityid`

因为不知道markdown的颜色如果设置，先单独提取出来，强调一下；欢迎高手markdown高手赐教！

      COALESCE(`air`.`aqi`,0)           AS `aqi`,
  COALESCE(`air`.`level`,'')         AS `level`,
  COALESCE(`air`.`position_name`,'') AS `position_name`